The pressure swing adsorption process (hereinafter referred to as PSA process) is widely used as a gas separation and refining process, and is used in a variety of application fields, such as removal of water or carbonic acid gas from air or nitrogen gas, production of hydrogen from steam reforming gas, such as butane, naphtha or methanol, production of hydrogen from coal-based generated gas, such as COG, recovery of hydrogen from gas produced by oil refining, and recovery of helium from natural gas.
With the use of an adsorptive separation apparatus employing a PSA process, the essential factors from the view point of consumers of product gas are high purity and recovery rate of product and a small variation in product flow rate and pressure.
In general, according to the PSA process which separates an unadsorbed component as high purity product, for regeneration of an adsorbent, unadsorbed component gas or product gas is used to purge an easy adsorptive component gas. The most primitive process is to use product gas stored in a tank, as an unadsorbed component gas for this regeneration. In this case, however, since the product gas is used for regeneration, the recovery rate of the product gas significantly decreases, resulting in an increase in the cost of products. In addition, since part of product gas is used at the time that an adsorbent bed purged for regeneration of an adsorbent is repressurized, there arises a problem of causing a variation in flow rate and pressure of product gas to be sent to consumers.
As a solution to this problem, a selective adsorptive separation process is known which is disclosed in, for example, Japanese Patent Publication Nos. 45-20082 and 55-12295. According to the disclosed processes, in principle, four adsorbent beds are provided. High pressure unadsorbed component gas remaining in the first bed whose adsorption step has been completed is used to increase the pressure, by pressure equalization, in the second bed which has already been purged and purified under lower pressure (e.g., atmospheric pressure). Intermediate pressure unadsorbed component gas remaining in the first bed is expanded and then used to purge the third bed under the lowest pressure to thereby prevent loss of precious product gas and improve the recovery rate. High pressure product gas is constantly introduced to increase the pressure in the fourth bed which has been purified from intermediate pressure to high pressure. It is described that the above solves the problem of causing a variation in flow rate and pressure of the product gas.
FIG. 3 illustrates a flow sheet described in the aforementioned Japanese Patent Publication No. 45-20082, and FIG. 4 a time schedule for the process.
Although the notation is partially altered for the ease of comparison with the present invention, the same reference numerals are used for the same elements to facilitate the comparison with the publication. Although the cycle time is changed, it should be properly set depending on the size of adsorbent beds, the type of an adsorbent, the flow rate of each gas, etc. and is substantially the same as the procedure disclosed in the publication.
Four adsorbent beds A, B, C and D are arranged in parallel between a raw gas feed manifold 10 and a feed manifold 11 for product gas or unadsorbed component gas. Automatic valves 1A, 1B, 1C and 1D serve to feed the raw gas from the raw gas feed manifold 10 to the adsorbent beds A, B, C and D, respectively, and automatic valves 2A, 2B, 2C and 2D serve to feed product gas consisting of unadsorbed component gas from the respective adsorbent beds to the product gas feed manifold 11.
Easy adsorptive component gas adsorbed in the individual adsorbent beds is removed and discharged through countercurrent depressurization and purging steps via a waste gas manifold 12 from automatic valves 3A, 3B, 3C and 3D provided on the side of the raw gas inlet of each adsorbent bed.
A conduit 15 for connecting between product gas discharge ends of the adsorbent beds A and B, an automatic valve 4AB and a throttle valve 17 serve to execute pressure equalization between the adsorbent beds A and B. Similarly, a conduit 16 for connecting between product gas discharge ends of the adsorbent beds C and D, an automatic valve 4CD and a throttle valve 18 serve to execute pressure equalization between the adsorbent beds C and D. Automatic valves 5A, 5B, 5C and 5D are disposed to connect the product gas discharge ends of the individual adsorbent beds, and two of these valves are simultaneously opened so as to permit the passage of cocurrently depressurized gas from one adsorbent bed so the gas is used for purging another adsorbent bed.
This cocurrent depressurization is neither performed between the adsorbent beds A and B nor between the adsorbent beds C and D, but should be done through purging conduits 21 and 22. Throttle valves 19 and 20 provided at the purging conduits 21 and 22 serve to prevent an excessive flow rate, and pressure regulators 23 and 24 are set to keep those adsorbent beds which are performing cocurrent depressurization, at the minimum pressure. When the adsorbent beds reach this set pressure, the pressure regulators 23, 24 are closed, thus terminating the cocurrent depressurization and purging steps.
An automatic valve 26 for main waste discharge and throttle valve 25 are provided in parallel on the waste gas manifold 12, and restrict the amount of waste gas at the time of countercurrent depressurization. In a countercurrent depressurization step in which the pressure in the adsorbent beds has not reached down to the lowest pressure, the automatic valve 26 is closed and the waste gas is permitted to pass only through the throttle valve 25 to thereby control the flow rate or the depressurizing speed in the adsorbent beds. In a purging step in which the pressure in the adsorbent beds is at the lowest level, the automatic valve 26 is opened to minimize the flow resistance of the waste gas.
A repressurization conduit 27 provided with a flow regulator whose opening and closing are controlled by a flow controller FC connects the product gas feed manifold 11 and the product gas discharge ends of the individual adsorbent beds through automatic valves 6A, 6B, 6C and 6D respectively belong to the adsorbent beds. The automatic valves belonging to those adsorbent beds which are in the step of increasing the product gas pressure, are opened so that a constant flow rate of product gas in the feed manifold 11 is introduced in the adsorbent beds.
A description will now be given of the individual steps according to the time schedule shown in FIG. 4.
When the adsorbent bed A is in the adsorption step, the automatic valves 1A and 2A are opened to introduce the raw gas to the adsorbent bed A from the raw gas feed manifold 10. As it passes through the adsorbent bed A, easy adsorptive component gas is adsorbed by an adsorbent in the bed and the concentration of unadsorbed component gas gradually increases, thus providing high purity product gas at the product gas discharge end of the bed. This product gas is sent to the product gas feed manifold 11 from the automatic valve 2A. This adsorption step continues for 5 minutes.
When this adsorbent bed A is in the adsorption step, the first 0.5 minute of the adsorbent bed B is for the countercurrent depressurization step where the automatic valve 3B is opened so that gas existing under pressure of 5 ata in the bed is discharged through the throttle valve 25 to decrease the pressure in the bed to 1.1 ata. In this step, part of easy adsorptive component gas adsorbed is desorbed and discharged. Upon elapse of 0.5 minute, the automatic valves 5B, 5C are opened, and after equal depressurization step, gas existing at 9 ata in the adsorbent bed C is expanded to 1.1 ata through the flow regulator 24 and the throttle valve 20. This gas then flows through the adsorbent bed B, accompanied with further desorbed easy adsorptive component gas adsorbed in the bed, and is discharged out from the system through the automatic valve 3B and newly opened automatic valve 26. During this period, the adsorbent bed B is in the purging step and the adsorbent bed C is in the cocurrent depressurization step. This status continues for 4.5 minutes.
The adsorbent bed C is in the equal depressurization step for the first 0.5 minute, and the pressure in the bed which has been 16 ata at the end of the adsorption step is equalized with the pressure of 1.1 ata in the adsorbent bed D so that both become 9 ata. This equalization is carried out by opening the automatic valve 4CD to cause the throttle valve 18 to adjust the flow rate to the proper level For the subsequent 4.5 minutes, the aforementioned cocurrent depressurization step is performed, and the automatic valve 4CD is closed and the automatic valve 5C is opened.
The adsorbent bed D, for the first 0.5 minute, is in the equal pressurization step where its pressure is equalized with that of the adsorbent bed C as described earlier; during the same period, the automatic valve 6D is opened at the same time to permit product gas from the product gas feed manifold 11 to flow through the conduit 27 and flow regulator 28 to be introduced into the adsorbent bed D. Upon elapse of 0.5 minute, although the equalization is completed and the automatic valve 4CD is closed, the automatic valve 6D remains open so that the pressure in the adsorbent bed D is increased by the product gas from 9 ata to 16 ata. This state continues for 4.5 minutes and the pressurization of the adsorbent bed D is completed.
The above describes the gas flow and pressure change at the individual sections in one cycle time of 5 minutes. Upon elapse of 5 minutes., the adsorbent bed D which has already completed the pressurization enters the adsorption step, and the steps of the individual adsorbent beds are changed as indicated in the time schedule shown in FIG. 4; after 4 cycles (20 minutes), the individual adsorbent beds complete the entire steps and then repeat the procedure again.
The aforementioned Patent Publication No. 45-20082 describes that the recovery rate of product gas can be improved and a variation in flow rate and pressure of the product gas can be reduced by constituting the pressure swing adsorption apparatus in the above manner and executing the individual steps according to the mentioned time schedule.
The reasons for the above are given in the Patent Publication No. 45-20082 as follows.
In the case where the adsorbent bed A is in the adsorption step, for example, the unadsorbed component gas of the adsorbent bed C under high pressure is moved to the adsorbent bed D which is under the lowest pressure until the former pressure becomes intermediate pressure through pressure equalization to the pressure of the adsorbent bed D. The unadsorbed component gas still remaining in the adsorbent bed C is sent to the adsorbent bed B for purging this bed B for regeneration. The reason why the recovery rate of the product gas can be improved is that precious product gas is not used for purging for a regeneration purpose.
The adsorbent bed D then has intermediate pressure through the aforementioned pressure equalization to the pressure of the adsorbent bed C. To increase the pressure to a higher level, product gas is introduced through the conduit 27 and flow regulator 28 to the adsorbent bed D. It is described that since this flow regulator 28 always permits the flow of a constant amount of product gas, it does not vary the flow rate and pressure of product gas sent to consumers.
However, the above process still has the following shortcomings.
First of all, since a significant amount of product gas is required to increase the pressure in the adsorbent bed D to high pressure from intermediate pressure, the improvement of the recovery rate is limited. Although, in the Patent Publication No. 45-20082, two stages of pressure equalization step are used to increase the intermediate pressure at the time of equalization to thereby prevent loss of product gas, the two-staged pressure equalization requires a longer cycle time so that the necessary amount of adsorbent increases, thus affecting the sizes of the adsorbent beds or the overall apparatus, etc.
Secondly, introduction of product gas through the flow regulator 28 is said to continue until the pressure in adsorbent bed D is repressurized approximately to the product gas pressure. Since this flow regulator 28 serves to control the flow rate at a constant level, however, the opening of the flow regulator 28 becomes greater as the pressure in the adsorbent bed D gradually increases from the intermediate pressure, i.e., the difference between both pressures becomes smaller. And upon completion of the repressurization, the flow regulator seems to be fully opened. The rate of the flow through this flow regulator 28 is controlled at a constant level and in such a way that the pressurization of the adsorbent bed is completed within one cycle time (in which one adsorbent bed is in the adsorption step); however, this is in practice hardly possible. This is because that according to the PSA process, when the demanded amount of product gas is changed, the amount of raw gas to be supplied is changed accordingly, i.e, that a so called operational load control is executed in a wide range. This control is basically carried out by changing the cycle time; when the load is increased, the amount of supply of the raw gas is increased to shorten the cycle time while with a decrease in load, the amount of supply of the raw gas is decreased to elongate the cycle time, thus varying the amount of gas per unit time.
Since the aforementioned repressurization of an adsorbent bed should be completed within a cycle time, the set flow rate of the flow regulator 28 should be made smaller if this time is long while it should be made greater if the cycle time is short.
Given that the maximum amount of load in the PSA process is 100%, the cycle time at the time of reduced operation under 20% load would become 5 times greater, so that the set flow rate of the flow regulator 28 would be 1/5. Since the opening of the flow regulator 28 gradually increases with an increase in pressure in the adsorbent bed D which should be increased, this in combination of alteration of the set flow rate by controlling the load requires an operation in a wide uncontrollable range.
Accordingly, the pressurization may not be completed within the cycle time or may be completed too early. In the former case, raw gas rapidly flows in the adsorbent bed which has not completed the pressurization at the time of bed switching, causing a pressure drop to thereby reduce the amount of product gas sent. In the latter case, however, due to earlier completion of the pressurization, there may be a period of time until the next switching in which no product gas flows through the flow regulator 28, and during which time the pressure and feeding amount of product gas would increase.
It is therefore an object of the present invention to provide, in a PSA process for separating product gas as hydrogen and/or helium, an adsorptive separation process which can improve the recovery rate of product gas and can further reduce a variation in pressure and flow rate of product gas.